Linear alpha-olefin is widely used in important commercial substances such as comonomers, detergents, lubricants, plasticizers or the like, and in particular, 1-hexene and 1-octene are commonly used as comonomers for controlling density of polyethylene during preparation of linear low density polyethylene (LLDPE).
In the conventional preparation process of LLDPE (Linear Low-Density Polyethylene), copolymerization of ethylene with alpha-olefin, for example, a comonomer such as 1-hexene and 1-octene is carried out in order to control density by forming branches in the polymer backbone.
Therefore, there is a problem that the production cost of LLDPE having a high content of comonomers is high. Various methods have been tried to solve this problem.
Further, because the application field or market size depends on the type of alpha-olefin, a technique capable of selectively producing a particular olefin is commercially important. Recently, many studies have been conducted on chromium catalysts for preparing 1-hexene or 1-octene with a high selectivity through selective ethylene oligomerization.
The conventional commercial methods for preparation of 1-hexene or 1-octene are the SHOP process of Shell Chemical, and the Ziegler Process of Chevron Philips, which are used to produce alpha-olefins with a wide distribution ranging from 4 to 20 carbons.
A chromium-based catalyst for ethylene trimerization having a ligand of the formula (R1)(R2)X-Y-X(R3)(R4) has been suggested, in which X is phosphorus, arsenic or antimony, Y is a linking group such as —N(R5)-, and at least one of R1, R2, R3 and R4 has a polar substituent or an electron donating substituent.
Further, studies have been conducted on (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2 as a ligand which shows no catalytic activity for 1-hexene under catalytic conditions and has no polar substituent in at least one of R1, R2, R3 and R4 (Chem. Commun., 2002, 858).
However, the prior ligands containing heteroatoms as described above are still required to maintain their polymerization activity consistently during reactions for producing 1-octene or 1-hexene and to have high selectivity.
Meanwhile, it is required to minimize production of by-products other than 1-octene to obtain pure 1-octene upon preparation of 1-octene by olefin oligomerization. However, production of by-products other than 1-octene is unavoidable in practice. Since the kind and content of by-products vary depending on the oligomerization catalyst, catalysts with high 1-octene selectivity must be used. However, some by-products may be incorporated in the polyolefin during polymerization and may result in improvement of physical properties. In this case, there is no need of removing by-products during purification of oligomerization products.
Accordingly, the present inventors have studied various catalyst systems for olefin oligomerization, and found that a catalyst system for olefin oligomerization as described below is used to improve 1-octene selectivity and to produce by-products capable of improving physical properties of polyolefins when 1-octene is used as a comonomer in the preparation of polyolefins, thereby completing the present invention.